You might think this is funny, but lower the rear seats, and leave the window in, then open and slam the mid gate... It has some pretty stiff latches
Good advice on checking the midgate. However, I've heard (and verified it based on experience) that slamming the midgate does not always latch it properly.
There are two latches at the top of the midgate, and they are double latches just like on the doors. Close it gently, and the first latch clicks and prevents it from falling, but it isn't pulled in tight to the weatherstipping. Close it harder, and the second latch clicks, and it's fully seated.
Slamming it might get both latches in the second position, or it might only get one. To be sure, close the midgate, and then push firmly on each latch: put the heel of your palm on one of the protruding bulges at the top of the midgate and press firmly: put your weight into it. Then repeat on the other side.
To verify that you are doing it hard enough, close the midgate gently, just until it clicks. If you try to move it now, you'll see it has some play. You are now in the first latch poisition. Now try the pushing trick, and you should hear a distinct click from each latch as it goes into the second latch position.
Once you are familiar with how much pressure it takes, closing it is a simple matter of flipping up the gate and then pushing on each latch. Maybe slamming it will get you there, maybe not. It's best to get in the habit of always checking (especially when you have noise or a leak.)